At the present time, there are relatively few sparsely populated areas worldwide where electrical power is not universally available through public or private utility companies which provide and maintain electrical power distribution network infrastructures. Power is supplied to these networks by generators reliant on fossil fuels or nuclear energy or from so-called renewable resources such as hydroelectric generators, wind turbines or solar collectors; all of which derive power directly or indirectly from heating or radiation effects of the sun. While generators using fossil or nuclear fuels can be located with substantial flexibility, since required fuels can be transported to them, generators using renewable energy sources must generally be placed at locations having environmental conditions favorable to collection of energy, including renewable biologically produced fuels since the energy density of plant material is generally much lower than for fossil fuels. In any case, it is generally the practice to locate power generators, regardless of type, in reasonable proximity to the locations where the major portion of the power that is produced will be consumed, such as near population or industrial centers, since a degree of power loss in the course of transmission is unavoidable, but sufficiently remote therefrom to avoid significant environmental impact to the extent possible. For this reason, a given network will generally be comprised of a portion for distributing large amounts of power at high voltage and with a plurality of phases (generally three) over substantial distances and numerous, more localized portions for distributing power at lower voltages and usually single-phase.
Since demand for power can be highly variable, it has also been the practice to interconnect power distribution networks and portions thereof through a so-called power grid or, simply, “grid”, serving wide geographic areas so that power demand can be averaged over greater numbers of consumers. That is, if the demand for electrical power in a given area exceeds the power generations capability of that area, power can be supplied to the area from an adjacent or even remote area where some excess power generation capacity exists. The interconnection of networks and network portions into a grid which can automatically transfer power between networks in accordance with demand, of course, requires all of the networks to be synchronized, generally using phase-locked loops (PLLs) controlling power converters and voltage source converters (VSCs) which can transfer power bi-directionally and provide variable power factor regulation which implies much increased complexity in the construction, operation and maintenance of such a grid. Further, disconnection of any network from the grid may cause a wide variety of unpredictable effects due to transients, resonances and the like that may appear when the topology of the grid is altered by disconnection of one or more networks from the grid. Such a disconnection is referred to as “islanding”; connoting a separation of a network or portion of a network from the main body of the grid. However, islanding also includes other operational abnormalities that extend over some determinable geographic region.
When an islanding event occurs and is detected, a network or network portion is not necessarily de-energized since the network or network portion may include one or more power generation sources or remain potentially connected to the grid through redundant power converters. Therefore, islanding events must be quickly and reliably detected and standards for such detection, notably IEEE-1547.2003 which requires islanding event detection to be achieved, in some cases, within ten cycles of the alternating current on the grid, have been promulgated in the interest of safety of maintenance personnel and to avoid damage to the network, grid or various loads that may be connected to portions of the grid or network. Present islanding event detection arrangements may generally meet this standard but often require close to ideal conditions to be present on the networks and grids. Islanding event detection is necessarily difficult where power generation provided to local loads is normally and predominantly local. Therefore, the amount of power transferred from the grid to a local network is usually very slight. That is, the local load demand is principally supplied by a local VSC and grid current is usually negligible. Therefore, when an islanding fault occurs, the voltage and frequency determined by the local VSC is virtually unaffected since there is negligible interaction of the grid with the load and local system.
Specifically, passive islanding detection methods can theoretically and practically detect abnormalities of interest while functioning normally during all nominal grid and network operating modes but are susceptible to so-called non-detection zones (NDZs); operational conditions (e.g. matched converter/load interaction) of the grid of which passive islanding detection methods cannot detect islanding events. In single-phase systems, known active islanding event detection schemes have involved using distortions injected into the power distribution system (e.g. a network or network portion) to cause instability when the grid is disconnected or non-dominant or use distortions generated in the frequency and/or phase of the reference current or power pulsing or dead-time to detect islanding events. Such distortions, perturbations or disturbances of voltage and/or current in the power distribution system must be substantial with respect to the local demand (whether they are continuous, discontinuous, periodic, etc.) to be effective and can thus lead to local grid stability concerns or potentially cause damage to or improper operation of loads connected to the grid. Furthermore, such active methods do not prevent NDZs but only reduce their size, and if not correctly implemented, will not cause the system to detect an islanding event. Therefore, both passive and active islanding detection methods are far from ideal and allow for significant improvements in islanding detection.
An article entitled “Islanding Detection Using a Coordinate Transformation Phase Locked Loop” by Thacker et al., Power Electronics Specialists Conference, 2007, PESC 2007, IEEE, 2007, pp. 1151-1156, which is hereby fully incorporated by reference, presents a PLL of a grid-connected VSC that can be easily modified such that the estimated frequency of the PLL inherently becomes unstable during the initial transients of an islanding event in a three-phase power distribution system. Such a conditionally unstable system is advantageous for islanding detection since, if the instability is conditioned on the loss of grid connection, then the system will always become unstable when islanding occurs without need for any perturbations when the grid is connected. In that approach, a three-phase PLL will cause initial transients but will not continue to oscillate or have a guaranteed drift of frequency outside the NDZ. However, application of that system is limited to three-phase VSCs while large portions of power distribution grids and networks operate with single-phase power and remain subject to islanding and have required islanding event detection using the far from ideal active and passive detection schemes discussed above.